1. Field of the Invention
The invention relates generally to the field of data networks. More particularly, the invention relates to methods of relaying synchronization status messages (SSM) at remote synchronization supply units (SSU), and remote synchronization supply units (SSU) that can relay synchronization status messages (SSM).
2. Discussion of the Related Art
Synchronization is essential for telecommunications networks. In order to generate stable and precise synchronization signals timing signal generators (TSGs) or synchronization supply units (SSUs) are installed in central offices. This is strongly recommended by many US and international telecommunication standards. The concept of having timing signal generators or synchronization supply units that generate and distribute signals within a central office is called BITS (building integrated timing supply) or SASE (stand alone synchronization equipment).
Due to the large amount of network elements (NEs) distributed inside central offices, and the redundant synchronization paths that are required to transport signals within the building, the SSUs have to provide a considerable amount of output signals through the entire building. The most-commonly used synchronization signals in the US are the DS1 (1.544 Mb/s) and the composite clock (CC) signals.
Remote SSU shelves strategically located within the same building or in adjacent buildings help distribute synchronization signals through the central office in remote locations. These shelves are typically slaved to the unique master synchronization supply unit shelf via composite clock signals, although other reference signal types could be used. However, due to phase alignment requirements of the composite clock driven network elements, remote synchronization supply units must be synchronized using composite clock signals in order to keep phase integrity with the master synchronization supply unit and all through the whole central office. FIG. 1 shows a typical application of the synchronization supply unit master and remote concept within a central office.
On the other hand, network elements that are synchronized via DS1 signals that originate at a remote synchronization supply unit may require synchronization status messages (SSM). The remote synchronization supply unit should be able to provide synchronization status messages to these network elements. The synchronization status messages are determined by the master shelf and should be relayed by the remote synchronization supply units to all the network elements in the building. However, since the remote synchronization supply units are connected to the master shelves via composite clock lines, the synchronization status messages cannot be transmitted via composite clock signals. This creates a problem when remote synchronization supply units have to be phased aligned and need to distribute synchronization status message signals to the DS1 driven network elements.
A critical requirement for most network elements that use composite clock signals is phase alignment. Phase alignment is achieved when all the data bits (1 through 8) on all the composite clock signals within the central office have the same polarity at the same time. For a remote synchronization supply unit it means that all its composite clock outputs must be phase aligned with its composite clock reference inputs. The composite clock signal specification is defined by Telcordia (Bellcore) GR-378-CORE and TR-TSY-000458 for signal type, amplitude, and wave shape, and is shown in FIG. 2.
Synchronization supply unit composite clock signals do not carry any useful information. All data bits of a composite clock signal are set to “1” to facilitate clock recovery on the other end. This feature makes the composite clock signal useless to carry any additional information, such as a synchronization status message to other synchronization supply units.
Synchronization status messages are messages that indicate the quality of the synchronization signal within a network. These messages can pass from one network element to another and, if properly used, can avoid timing loops when reconfigurations happen within a network. They also can be a useful tool for monitoring and maintaining the health of the synchronization signals that travel within a network.
In DS1 signals synchronization status messages travel in the data link (DL) bit of the ESF framing and consists of a 16-bit codeword. These messages are defined by different entities like ANSI TR#33 and Telcordia (Bellcore) GR-378 in North America.
FIG. 3 shows a typical master synchronization supply unit system. It accepts one or more input reference signals, which can be DS1, CC, or any other type. The DS1 signals can carry synchronization status messages as well. The master synchronization supply unit has internal high stability clocks, which help filtering out jitter and wander. These clocks provide holdover capability in the event that the reference signals are lost. This capability allows the network element connected to the clock to run for a certain period of time without losing synchronization. An output section generates different types of signals required by the network elements. The signals can be DS1, CC, or any other type including analog type of signals. The DS1 signals can contain synchronization status messages, which are broadcast to all the network elements connected to the master synchronization supply unit.
A remote synchronization supply unit is similar to a master synchronization supply unit (FIG. 3), except that the input signals are typically composite clock only, and may or may not have clocks. This remote synchronization supply unit is connected to any output signal of the master synchronization supply unit. Many remote synchronization supply units can be connected in a star topology to the master SSU. (FIG. 1)
Since the composite clock output signals should be phase aligned with the input reference signal, the clocks should not have any time constants that add some phase delay. These clocks provide a minimal holdover capability. If phase alignment is not a requirement, the remote shelf can accept DS1 or any other type of signal instead.
If composite clock phase alignment is required, the synchronization status message generated by the master synchronization supply unit, or any master synchronization source, cannot be relayed by the remote synchronization supply unit, since the composite clock signal does not provide any means of carrying synchronization status message data.
Current remote shelves do not support synchronization relaying capability, since there are no means of transmitting SSM information over the CC lines to the remote shelf. Typical connections to remote shelves are done using CC signals only, which as previously explained, do not carry this information, thus making the broadcast of SSM messages impossible.
A less common and seldom used method relies on management systems that reads the current SSM status from the master shelf and then send a command to the remote shelf, setting the appropriate SSM message. However, this method depends on an external management system and a specialized software, as well as connections links to the master and remote shelves, making it more expensive, more system manufacturer dependent and less reliable.
Heretofore, the need for relaying a synchronization status message to a network element by a remote synchronization supply unit has not been fully met. What is needed is a solution that addresses this need.